Power control and soft handover are two important functionalities of the Code Division Multiple Access (CDMA) air interface. Power control is used to continuously adjust transmission power such that the perceived quality is sufficiently but not excessively good. By controlling transmission power in this manner it is ensured that not more than necessary interference is generated, which in turn results in that the system performance can be improved.
Typically, there are two power control loops, the inner loop and the outer loop. The inner loop power control adjusts transmission power so that the Signal to Interference plus Noise Ratio (SINR) perceived by the receiver is close to a SINR target set for the receiver. The inner loop power control is typically implemented by sending power control commands via so called Transmission Power Control (TPC) bits. The power control command is obtained at the transmitter by comparing the received SINR with the SINR target. To increase the transmission power the TPC bit indicates “up” and to decrease the transmission power the TPC bit indicates “down”. The uplink power control command is sent via downlink dedicated physical control channel (DPCCH) for High Speed Downlink Package Access (HSDPA) incapable User Equipment (UE) and associated dedicated physical channel (A-DPCH) or fractional dedicated physical channel (F-DPCH) for HSDPA capable UE. The downlink power control command is sent via uplink DPCCH.
The outer loop power control adjusts the SINR target so that the desired Quality of Service (QoS) requirement is met.
During a soft handover, a mobile station also termed user equipment (UE) is in the overlapping cell coverage area of more than one base station. The communication between the mobile station and base stations takes place concurrently via all connected radio links. Soft handover is a way to decrease the dropping probability when a mobile station moves from one cell to another cell.
During the soft handover, one mobile station is connected to two or more base stations and is power controlled by all the base stations to which the mobile station is connected. Each connected base stations transmits a power control command to the mobile station. The mobile station typically combines the received power control commands from all the base stations in the following way:                A power increase only if all the power control commands indicate “up”        Otherwise the power is decreased        
As a result of the above way of combining inner loop power control commands a mobile station in soft handover will more likely decrease its transmission power when the received power control commands are unreliable since any unreliable power control command that is misunderstood as “down” will lead to that the mobile station decrease the transmission power. Hence, the transmission power may be decreased when it should have been increased. Too low uplink power can lead to bad uplink quality.
To solve the problem of risking a bad uplink quality, a TPC discarding mechanism can be employed, as is described in Niclas Wiberg, Hu Rong, Fredrik Gunnarsson, Bengt Lindoff, “Combining of Power Control Commands During Soft Handover in WCDMA”, PIMRC2003. Hence, the reliability of received TPC bit can be checked before power control commands are combined. Unreliable TPC commands, i.e. the TPC commands with error probability greater than a predefined threshold, are discarded and only the reliable TPC commands are combined. In the case when all TPC commands are discarded, the transmission power is kept the same, neither increases nor decreases, which is referred as “hold”.
The TPC discarding mechanism solves the problem of risking a too low uplink quality caused by unreliable TPC commands. However, the TPC discarding mechanism may cause other problems.
One example of such a problem is illustrated in FIGS. 1a-1b. In FIGS. 1a and 1b, performance and measures when one Enhanced Uplink (EUL) User Equipment (UE) is in soft handover are shown (the UE is also HSDPA capable and uses A-DPCH in downlink). The plots in FIG. 1a are the rise over thermal (RoT) averaged over one sub-frame in the serving and the non-serving cells of the UE.
In FIG. 1b, the upper plot is the SINR of the best A-DPCH in the Active Set (AS) while the lower plot of FIG. 1b is the SINR of the second best A-DPCH in the AS. The TPC power offset relative to the pilot bits carried by A-DPCH is 3 dB.
In the lower plot of FIG. 1b it can be seen that at times the SINR of the second best A-DPCH falls below the TPC discarding threshold. The fact that the second best A-DPCH falls below the TPC discarding threshold leads to a situation where the TPC command carried on that dedicated physical channel (A-DPCH) is discarded by the UE when the discarding mechanism is used. The TPC is regarded as unreliable because the SINR of the A-DPCH is below the TPC discarding threshold 0.5 or −3 dB, which corresponds to a certain TPC error rate.
In the case when the TPC that is discarded indicates “down” while the TPC that is not discarded indicates “up”, uplink power will be increased unnecessarily. The excessive increased uplink power results in that the received uplink SINR become higher than the uplink SINR target. This can in turn lead to high RoT peaks, as shown in FIG. 1a and cause the system risks becoming unstable.
This is a typical situation which is often referred as unbalanced uplink and downlink. Usually a soft handover UE does not stay in the situation with unbalanced uplink and downlink for a long time period, and the uplink/downlink unbalanced situation occurs only for a very short time period, as shown in FIG. 1a where one high RoT pulse appears after 3 s.
However, a high RoT pulse will also cause an uplink power rush for users who are not in soft handover. Such a high and short RoT pulse is difficult for Uu load control to reduce via a Node B scheduler since there are measurement and operation delays.
One solution is to increase the reliability of TPC by setting a higher quality target for the relevant downlink channel carrying the uplink TPC command, for example the A-DPCH.
Another solution is to increase the TPC transmission power so that the TPC is more reliably received. The transmission power on TPC bit can be increased by increasing the TPC power offset, which is supported in 3GPP. The TPC power offset can be varied from 0 dB to 6 dB, see 3GPP TS 25.331, “Radio Resource Control (RRC)”, v.7.4.0. The higher the TPC power offset, the lower the TPC error rate. Dynamical setting of TPC power offset based on some relevant quality is supported in 3GPP, see 3GPP TS 25.214, “Physical layer procedures (FDD)”, v.7.4.0 by signaling the quality measurement, see 3GPP TS 25.433, “UTRAN Iub interface Node B Application Part (NBAP) signaling)”, v.7.5.0. Also, in WO2006/081874 a method to adjust the TPC power offset was proposed, where the TPC power offset is increased based on an event triggered report from user in the case when a number of M unreliable TPC commands are registered during a given time period.
However, to set a higher quality target for the relevant downlink channel has drawbacks. It requires a higher downlink power consumption on the channel used, which causes a higher downlink interference. This will also negatively impact the downlink performance. To set a higher TPC power offset can mitigate the uplink power rush problem to some extent. In the case when a soft handover is used and a user has more than two radio links in the active set while uplink and downlink are unbalanced, the uplink TPC discarding probability can be high for one of the downlink radio link set with relatively bad quality.
Hence, there exists a need for a method and a system that improves over existing techniques for transmitting and using TPC commands.